Oviparous

eggs fertilization sperm externally

Oviparous is a zoological term that refers to animals that lay eggs which then hatch externally.

Oviparous animals may fertilize their eggs either externally or internally. External fertilization involves the passage of the sperm to the ova through an ambient medium, usually water. For example, frogs achieve external fertilization of their eggs during amplexus, when the male deposits sperm over the eggs as they are laid by the female. External fertilization in many aquatic invertebrates is less well controlled; for example, in the case of many marine invertebrates which shed immense numbers of gametes to the water more or less simultaneously, with ova and sperm meeting somewhat by chance. All animals that fertilize their eggs externally are oviparous.

In cases of internal fertilization, male animals somehow pass their sperm into the female. For example, male salamanders deposit a sperm packet, or spermatophore, onto the bottom of their breeding pond and then induce an egg-bearing (or gravid) female to walk over it. The female picks up the spermatophore with the somewhat prehensile lips of her cloaca, and retains it inside of her body where the eggs become fertilized. These fertilized eggs are later laid and develop externally, representing oviparity.

Many species of fish, most species of lizards and snakes, all species of crocodilians and birds, and even certain primitive mammals such as the platypus and echidnas achieve internal fertilization of their eggs through copulation. If the eggs are then laid to develop externally, the process represents oviparity.

Ovoviviparity involves retaining of the fertilized eggs in the body of the female until they hatch, so that "live" young are born.

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